In the area of electronic parts, dielectric and magnetic materials play important roles, the former being popularly used as materials mainly for capacitors, and the latter being popularly used as materials mainly for inductors. The capacitors and inductors are manufactured from the respective materials.
An EMI filter conventionally used for a signal line, for example, is manufactured in general by combining individual elements of the capacitor and inductor as described above. It has, however, been impossible to achieve downsizing of the product with the conventional EMI filter composed of a combination of the individual elements of a capacitor or an inductor.
To solve this problem, it has conventionally been tried to manufacture an EMI filter combining the functions of capacitance with those for inductance by using a composite ceramic material prepared by mixing dielectric and magnetic materials and sintering the mixture.
Sintering a mixture of dielectric and magnetic materials at a high temperature has however led to a reaction between the dielectric and magnetic materials. This has resulted in difficulties such as deterioration of the respective properties and impairment of the sintering reproducibility, as well as deterioration of the EMI filter properties and impairment of reproducibility.
Various composite LC parts and composite LC chip parts having a monolithic structure combining an inductor and a capacitor have been proposed. For example, the present inventors proposed a composite LC part manufactured by forming a laminate for a capacitor comprising alternately printed and laminated dielectric ceramic to layers and conductors for the capacitor, forming a laminate for the inductor comprising alternately printed and laminated ceramic layers of a magnetic material and conductors for the inductor on this laminate for the capacitor or separately therefrom, integrally sintering these laminates for the capacitor and laminates for the inductor interposed with an intermediary layer in between, and providing an appropriate external terminal at an end of the laminate having the exposed inner conductors therein (Japanese Laid Open Patent Publication No. 3-166,810, for example). In the laminate for the inductor of this composite LC part, a helical inductor is formed vertically to the layers, i.e., in the thickness direction, by connecting each layer of the conductors for the inductor formed on the ceramic layers via a hole through the ceramic layers.
As another composite LC part, the present inventors developed a .PHI.-type LC filter, in which a laminated chip inductor comprising a sintered ferrite having an inner electrode provided therein and a laminated chip capacitor comprising a sintered dielectric material having an internal electrode and a grounding electrode are integrally combined with an adhesive, and the external electrodes of the laminated chip inductor and the laminated chip capacitor are electrically connected (Japanese Laid Open Patent Application No. 6-325,977).
Furthermore, another composite LC part is proposed, in which the ceramic layers forming the laminate for the inductor and the ceramic layers forming the laminate for the capacitor are prepared from ceramic dielectric materials. The laminates for the inductor and the capacitor are integrally sintered in a superposed state, and an appropriate external terminal is provided at an end where an internal conductor is exposed.
However, in the composite LC part disclosed in Japanese Laid Open Patent Publication No. 3-166,810, the difference in the coefficient of thermal shrinkage between the lamination for the capacitor and the lamination for the inductor causes heat stress during sintering. This results in unavoidable strain, cracks and fluctuations of the properties even in the intermediate layers. This produces inferior yield and reliability of the product.
In the .PHI.-type LC filter disclosed in Japanese Laid Open Patent Application No. 6-325,977, which is free from the defects as described above, it is necessary to separately sinter the laminated chip inductor and the laminated chip capacitor, and control of the production of the chip inductor and the chip capacitor is complicated and troublesome, which makes it difficult downsize the product.
In the composite LC part formed exclusively from a dielectric material the inductance available is insufficient when used for noise removal, and since all the windings of the coil pattern forming the inductor always contain the dielectric material in between, a large floating capacity between the windings results which makes it impossible to obtain the desired performance.